Key Factors for Dense Copper Coating by HVOF Spraying

place in preferential to the corrosion of copper coating (Eq.
3) because the corrosion potential of these coatings were
approximately –400 mV and close to that of the free steel
plate at approximately –450 mV. On the other hand, the
corrosion reaction of coating (Eq. 3) takes place exclusively
and preferentially in case of not any and a slight amount of
penetrating pores, respectively. This is because the corrosion
reaction of steel substrate is negligible. This is the case for the
copper coated steels under the Cond. C and D because their
corrosion potentials corresponded to that of the free copper
plate around –130 mV.
These results revealed that the coating density at the
fuel/oxygen ratio over 0.66 (Cond. C and D) improved more
rapidly than that at the ratio below 0.56 (Cond. A and B). This
fact implies that a certain important factor exists to improve
the coating density, i.e. compactibility of sprayed particles at
the fuel/oxygen ratio over 0.66. Such a factor was discussed in
the next section.
Factors to improve compactibility
The compactness of HVOF spray coating was expected to
depend mainly on the transformation degree of sprayed
particles upon impinging, in other words, amounts of melted
particles and highly deformable particles.
Typical results of capturing spray particles by agar gels are
shown in Fig. 6. The spherical particles at the deeper positions
corresponded to unmelted particles, which plunged into the
agar gel without broken off. On the other hand, the fine
particles at the shallower positions corresponded to melted
particles, which were impinged to the gel target, broken into
fine particles and solidified, resulted in being captured near the
surface. As the fuel/oxygen ratio increases, the unmelted
particles at the deeper positions decrease whereas the melted
particles increase, as seen in Fig. 6. In order to show this
phenomenon quantitatively, molten fractions of the sprayed
particles were determined and represented in Fig. 7. At the
ratio of 0.46 under the Cond. A, the molten fraction was 0 wt%.
(
Abundance ratio (%
100
80
60
40
20
TYPE2
TYPE1
TYPE4
TYPE3
0
Cond. A B C D
TYPE5
Figure 10 Abundance ratios of five types splats under each
condition.
As the ratio increased, the molten fraction increased, especially
considerably at the ratio of 0.87 (Cond. D). This rapid increase
at the ratio of 0.87 (Cond.D) did not agree to the some
experimental results that the compactibility of sprayed particles
improved rapidly at the ratio of 0.66 (Cond. C), as mentioned
above. Accordingly, the molten fraction was not a primary
factor to improve the coating compactness under the HVOF
spray conditions in this paper although the molten fraction
must play an important role on making a dense coating.
The splat morphology is related to deformability of sprayed
particles. Figure 8 shows microscopic images of splats
prepared under four spray conditions. The splats could be
classified into some types of shapes such as lump, bullet
wound and splash. As the fuel/oxygen ratio increased, the
number of lump and bullet wound decreased whereas that of
splash increased. In order to estimate the sprayed particle
deformability quantitatively, three-dimensional information of
the splats was collected by the laser microscopy. According to
the height (or depth) and the area, the splats were classified
into five types of shapes, as shown in Figure 9. In this figure,
observed images are presented at the left side and their
schemas are illustrated at the right side. The five types of splats
are called as TYPE X for convenience below. The shape of
TYPE 1 looks like bullet wound and is supposed to be formed
by impinging of unmelted hard particles and by their falling
apart. TYPE 2 is considered to be part of such hard particles
sticking to the target. The shapes of both TYPE 3 and TYPE 4
were caused by the unmelted particles soft enough to deform
plastically and their deformation degrees may depend on
temperature and impact force of the spray particle. The shape
of TYPE5 is an extremely thin and flat and this is due to
melting of particles. About 100 of splats were sampled
randomly on the surface of test target sprayed under each
condition. The numbers of splats were counted according to the
above-mentioned classification and their abundance ratio was
determined and shown in Fig. 10. The increase in abundance
ratio of TYPE 5 is good agreement with the increase in molten
fraction of melted particles at the ratio of fuel to oxygen of
0.66 (Cond. D). This increase in the number of melted particles
may be related to the result that the higher value was obtained
especially under Cond. D on measuring the flight velocity of
sprayed particles (see Fig. 1). When Cond. B is compared to
Cond. C in terms of the abundance ratio, both TYPE 4 and
TYPE 5 increases under Cond. C. However, the increasing rate
for TYPE 4 is larger than that for TYPE 5. Therefore, the
reason why the compactibility of sprayed particles was
improved rapidly under Cond. C is the increase of the
unmelted particles soft enough to deform plastically, in other
words, with the sufficiently high plastic deformability upon
impinging.
Conclusion
In this study, we revealed that the key factors determining the
compactness of sprayed particles in HVOF sprayed copper
coatings. It is natural that the molten fraction of sprayed
particles is important and the higher fraction leads to form
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